I can't post the workbook because I don't have permission.
You can always create a sample workbook to replicate the scenario without divulging any confidential information. See Packaged workbooks: when, why, how
For your measure though, you could create a calculated field to exclude the month below a certain date, SUM(IF DATETRUNC('month',[Order Date])<#2016-12-01# then Sales end)
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